Question

A current of 5 mA flows through a 2 mm inner-diameter glass tube filled with 1.0 mol dm-3 NaCl(aq) solution in the anode compartment in addition to a CH3COONa (aq) solution in the cathode compartment. The boundary created between the two solutions moves 6.05 mm towards the anode in 10 minutes. Calculate the transport number of the chloride ion in the solution.

Answer 1

Generally, the transport number is calculated by two different methods and one of the method is called, Moving boundary method.

The transport number of a cation is calculated as,

t⁺ = z₊. d.l.F.c / Q.T

and therefore t^- = 1- t⁺, hence (t⁺ + t^- =1)

whereas, z₊ is the cation change (z=1),

            d – cross section or diameter of the tube (2.0 mm)

            l – distance moved by the boundary (6.05 mm)

            F – Faraday constant (96500 coulombs/mol)

            C – concentration of the solution (1.0 mol dm-3 = 10⁶ mol/mm-3)

            Q – the applied electric current (5 mA = 0.005 couloms/s)

T – boundary moved at a given time (10 min = 600 s)

Applying,

t⁺ = 1 * 6.05 * 2 * 96500 * 1.0 / 10⁶ * 0.005 * 600

                        = 0.3892

Since, t^- = 1 – t⁺

            t- = 0.6108

The transport number of chloride ion in the solution is = 0.6108